3 HAZARDOUS SUBSTANCES
3.1 INTRODUCTION
Nuclear production of hydrogen is associated with the use of several hazardous substances that increase concerns regarding the safety of the combined complex, especially since the thermochemical and hybrid technologies require that the nuclear plant be in close proximity to the chemical plant, mainly to reduce heat losses incurred during heat transfer. Therefore, the nearness and coupling of the plants introduce alternate heat, mass and radiologic paths of interaction within the combined complex. Moreover, while the safety philosophy of nuclear plants is containment of hazardous material, chemical facilities prefer a controlled release of confined material to the environment in order to mitigate potentially explosive situations (Forsberg et a!., 2007). These radically different safety philosophies and alternate paths of interaction influence the safety analysis of the technology and increase the importance of being thoroughly knowledgeable regarding the hazardous substances involved. These substances pose a flammable, corrosive, toxic, radioactive or asphyxiation hazard to the health of the operating personnel, integrity of the structures or safe operation of the plant and therefore form the first phase of a safety related investigation. The risks and physical- and chemical properties of the hazardous material associated with the previously discussed hydrogen production technologies subsequently form this chapter's topic of discussion. To this extent, hydrogen, methane, natural gas, carbon monoxide, carbon dioxide, sulphuric acid, sulphur dioxide, sulphur trioxide, iodine, hydrogen iodide, oxygen and helium are examined as hazardous materials in the following subsections. All of these aspects play vital roles in the successful implementation of a nuclear-assisted hydrogen production facility and require thorough investigation.
3.2 HYDROGEN (H2)
3.2.1 PHYSICAL PROPERTIES
Hydrogen is the lightest and most abundant element on earth and not detectable in any concentration to human senses. Above 20 K, hydrogen is a colourless, odourless, tasteless, non-acid and non-metallic diatomic gas. It is not toxic, but can result in asphyxiation by diluting the oxygen content of air. If the oxygen
concentration of air is less than 19.5 percent by volume, it is considered oxygen deficient (NASA, 2005).
Hydrogen has a very high energy density and 1 kg of H2 contains the energy
equivalent of 2.4 kg of methane and 2.8 kg of gasoline (petroleum). However, in many applications volume is a more suitable comparison and hydrogen becomes less attractive due to its very low density. In energy content, 1 L of liquid H2
corresponds to 0.27 L of gasoline, while 1 L of gaseous H2 at 350 bar and room
temperature is equivalent to 0.1 L of gasoline or 0.3 L of methane also at 350 bar (Piera eta/., 2006; Verfondem & Nishihara, 2004a).
Gaseous hydrogen at its BP (20 K) is heavier-than-air but becomes positively buoyant at temperatures above 22 ~ 23 K and exhibit a high diffusivity and buoyant velocity (IAEA, 1999). Upon release, hydrogen rapidly diffuses in the ambient air with the diffusion velocity proportional to the diffusion coefficient and varies with temperature according to T3/2. The rising velocity of gases under the influence of
buoyant forces is dependent on the density difference between the gases as well as on drag and friction forces. In addition, the shape and size of the rising gas volume and atmospheric turbulence influence the final velocity of the rising gas (BRHS, 2007). Hydrogen's high diffusion rate and buoyancy is a positive safety aspect in unconfined spaces but may be hazardous in partially confined to confined spaces where it can accumulate underneath an obstacle such as a roof or ceiling (Verfondem & Nishihara, 2004a). Even though the rapid mixing of hydrogen with air may quickly lead to the formation of flammable mixtures, it is a positive safety aspect since it will also quickly dilute to non-flammable mixtures. To this extent, it is estimated that during a typical unconfined hydrogen explosion only a fraction of the gas cloud mixture is involved, and consequently only a few percent of the theoretical energy content of the cloud is released (BRHS, 2007).
Hydrogen has a higher propensity to leak and diffuse through material due to its small size, small molecular weight and low viscosity. Diffusion in small amounts is possible through intact materials, especially organic materials, for both liquid and gaseous hydrogen. When compared to water and nitrogen, the leakage rate of hydrogen is fifty times higher than that of water and ten times that of nitrogen. At elevated temperatures and pressures hydrogen attacks mild steels severely, causing decarburization and embrittlement, which is a serious concern in any application of hydrogen involving storage or transfer of hydrogen gas under pressure. Hydrogen
embrittlement changes the mechanical properties of materials by changing its mechanical behaviour (making it brittle) and may result in vessel rupture and subsequent loss of containment. This aspect of hydrogen is more prolific at high temperatures and pressures, such as those associated with the majority of the nuclear hydrogen production technologies, but appropriate material selection and design can mitigate the extent to which it occurs (BRHS, 2007; Verfondern & Nishihara, 2004a; NASA, 2005).
The hydrogen molecule exists in two forms and is distinguished by the relative rotation of the nuclear spin of the individual atoms in the molecule. Molecules with spins in the same direction (parallel) are called ortho-hydrogen, while those with spins in the opposite direction (anti-parallel) are called para-hydrogen (NASA, 2005). The contribution of ortho- and hydrogen depends on temperature, with para-hydrogen dominating at low temperatures (99.825 % para at 20 K) and ortho-hydrogen at higher temperatures (75 % ortho at 25 °C, also known as normal hydrogen). The transition between the two states is very slow in the gas phase (0.0114/h) and takes place over a long period (approximately 3 to 4 days). However, magnetic impurities, a radiation field and the presence of oxygen can catalyze the ortho-para transition and increase the transition rate to hours (Verfondern & Nishihara, 2004a). The ortho-para-hydrogen conversion is accompanied by a release of heat (703 J/g at 20 K) for ortho- to para-hydrogen conversion and normal to para-hydrogen conversion (527 J/g). This heat is important considering that the latent heat of vaporization (446 J/g) is less than the energy of conversion, thereby requiring that appropriate safety and control mechanisms be in place for liquid hydrogen applications.
Hydrogen also exhibits a positive Thomson effect above its inverse Joule-Thomson temperature of 193 K, which means that the temperature of hydrogen increases when it expands from a high-pressure to low-pressure atmosphere above 193 K (NASA, 2005). However, since the temperature change due to this effect is relatively small (6 K for depressurization from 20 MPa to ambient pressure), the probability of spontaneous ignition solely by this effect is very small, except if the gas was already near to its auto-ignition temperature (NASA, 2005). An ignition is more likely to occur because of electrostatic charging of dust particles during depressurization (BRHS, 2007; Verfondern & Nishihara, 2004a).
3.2.2 CHEMICAL PROPERTIES
Mixtures of hydrogen and air, oxygen, or other oxidizers are highly flammable over a wide range of concentrations. The flammability limits, given in volume percent
hydrogen, specify the range over which the fuel vapours will ignite when exposed to a sufficiently strong ignition source and depend on the ignition energy, temperature, pressure, presence of diluents, and size and configuration of the containment or atmosphere (NASA, 2005). A stoichiometric hydrogen-air mixture contains 29.5 vol.% hydrogen such that it consumes all fuel upon combustion and releases the maximum amount of combustion energy. The combustion of hydrogen liberates the chemically bound energy as heat and produces water vapour as product according to the following reaction (Verfondern & Nishihara, 2004a):
H2 + \ 02 -»H2 O (-292.5 kJ/mol) Reaction 3-1
The flammability limits of hydrogen in dry air at 101.3 kPa and ambient temperature are 4.1 % (LFL) to 74.8 % (UFL), while the corresponding limits for hydrogen-oxygen mixtures are 4.1 % to 94 %. Figure 3-1 graphically represents the flammability limits of both hydrogen-air and hydrogen-oxygen mixtures at 101.3 kPa and 298 K (NASA, 2005;McCartyefa/., 1981).
Furthermore, a reduction in pressure below 101.3 kPa tends to narrow the range of flammability (H2/air mixture: LFL = 20-30 % at 6.9 kPa and 9 % at 10.3 MPa),
whereas an increase in temperature results in broadening of the flammability range (H2/air mixture: LFL = 7.7 % at the BP and 2.3 % at 723 K; NASA, 2005). In case of
emergencies, diluents and halocarbon inhibitors can be added to the mixture to narrow the flammability limits as are shown in Figure 3-2 and Figure 3-3. The effects of He, C02, and N2 as diluents on the flammability limits of hydrogen-air mixtures
were performed at NTP, while water vapour was performed at 422 K. Water vapour was the most effective in reducing the flammability range, and helium the least effective (NASA, 2005; McCarty etal., 1981).
Figure 3-2: Effects of N2j He, C02] and H20 Diluents on flammability limits of hydrogen
in air at NTP (NASA, 2005)
Figure 3-3 shows the effects of halocarbon inhibitors on the flammability limits of hydrogen-oxygen mixtures and the effectiveness of the inhibition differs significantly and will therefore depend on the specific system on which it is to be used (NASA, 2005; McCarty et al., 1981).
The flame temperature of hydrogen in air is 2318 K at the stoichiometric concentration and reaches a maximum of 2403 K in a 31.6 vol.% hydrogen-air mixture (IAEA, 1999). The burning velocity, defined as the subsonic velocity at which
a flame propagates through a flammable fuel mixture, is 2.55 m/s for a stoichiometric hydrogen concentration in ambient air, 3.20 m/s for a 40.1 % hydrogen-air mixture at ambient conditions, and increases to a maximum of 11.75 m/s in oxygen mixtures (NASA, 2005, Verfondern & Nishihara, 2004a; BRHS, 2007).
Figure 3-3: Effects of halocarbon inhibitors on flammability limits of hydrogen-oxygen mixtures at NTP (NASA, 2005)
The burning velocity of hydrogen is much higher than that of other hydrocarbon-air mixtures (eight times greater than that of natural gas-air and propane-air mixtures) due to hydrogen's fast chemical kinetics and diffusion velocity, and therefore the probability of a deflagration-to-detonation-transition (DDT) of a hydrogen-air mixture is higher (BRHS, 2007; DOE, 2004). The flame speed is much higher than the burning velocity because of the expansion of the combustion products, instabilities and turbulent deformation of the flame. The maximum flame speed for a deflagration of a stoichiometric hydrogen-air mixture is 975 m/s, which is the speed of sound in the unburntgas mixture (BRHS, 2007).
Hydrogen has a relatively high auto-ignition temperature, which can be lowered by catalytic surfaces and is only slightly higher in air than in oxygen. At 101.3 kPa the range of reported auto-ignition temperatures for stoichiometric hydrogen in air is 773 to 850 K, while it is approximately 773 - 833 K in stoichiometric oxygen (at 20 - 50 kPa gaseous hydrogen-air ignitions have occurred at 620 K). Since hydrogen is
already a gas at ambient conditions, it does not have a flash point, but cryogenic hydrogen will flash at all temperatures above its BP of 20 K (NASA, 2005; Verfondem & Nishihara, 2004a; BRHS, 2007).
The minimum ignition energy is the spark energy required to ignite the "most easily ignitable fuel concentration in air and oxygen" (usually not the stoichiometric mixture), and is 0.02 mJ for hydrogen, which is much lower than the corresponding values for flammable hydrocarbon-air mixtures (methane: 0.29 mJ, gasoline: 0.24 mJ). However, a weak spark or the electrostatic discharge by a flow of pressurized gas or by a person (~ 10 mJ) would be sufficient to ignite any of these flammable fuel mixtures (BRHS, 2007). Moreover, the minimum ignition energy decreases with increasing temperature, pressure and oxygen concentration. When ignited, the flame produced from a hydrogen-air-oxygen mixture is colourless and any visibility is due to the presence of impurities. However, a pale blue or purple flame may be observed at reduced pressures (Verfondem & Nishihara, 2004a; NASA, 2005; BRHS, 2007).
The maximum experimental safe gap (MESG) is the maximum distance between two flat plates that allows flame propagation and is 0.08 mm for hydrogen. The quenching gap in NTP air is the distance between two flat plates at which ignition of a flammable mixture is suppressed and corresponds to the smallest diameter of a tube through which a flame can propagate (BRHS, 2007). Faster burning gases generally have smaller quenching gaps and flame arresters of faster burning gases require smaller apertures (NASA, 2005). Hydrogen has a quenching gap of 0.64 mm in NTP air, but this value depends on the temperature, pressure, and composition of the combustible gas mixture and the electrode configuration (0.076 mm is the lowest reported value for hydrogen; NASA, 2005). The MESG is always smaller than the quenching gap due to high explosion pressures (BRHS, 2007).
The amount of heat radiated by a fire is very significant since it can initiate secondary fires, damage the surrounding buildings and harm (bum) nearby personnel. The average emissivity of a flame and the ambient water content are more important factors than flame temperature when considering the heat flux to the surroundings (DOE, 2004). Ambient water absorbs thermal energy radiated from a fire and therefore reduces the thermal radiation hazard. Hydrogen has a very low emissivity (e = 0.1) and therefore radiate less heat than natural gas, propane, gasoline, and kerosene flames (e ~ 1), although the flame temperatures are comparable. Comparatively, the burning hazard of a hydrogen fire is significantly smaller than that
of combustible hydrocarbon fires (Verfondem & Nishihara, 2004a). According to NASA (2005), the absorption of thermal energy by ambient water is very significant for hydrogen fires. Furthermore, the intensity of radiation from a hydrogen flame at a specific distance depends heavily on the amount of water vapour present in the atmosphere and is expressed by the following equation (NASA, 2005):
Le -0,0046VIT Equation 3-1
Where:
I Intensity [J/cm2.s]
h
Initial intensity [J/cm2.s]W Water vapour [weight % r Distance [m]
Figure 3-4 shows the variation in distance from a hydrogen fire for a thermal radiation exposure of 2 cal/cm2 and duration of 10 s while noting that 2 cal/cm2 (8.4 J/cm2) is
approximately the radiant flux required to produce flesh burns and ignite certain combustible materials in short exposure times (NASA, 2005; McCarty et a/., 1981).
1T«ter Vapor, percent
1 0 5 r: / / - • "* ._ \ - .* S _«.._.- 2 7 / f
3
1 0 * - / r < i f PI -0) be f O 7 i • _ , _ - — ■ ■ h ** ———" fc-, 1 0 3 Z/ s
^ — - —
W ; / / _-—-"—"""" >M - y r* ^ o - / j ^ 03 _ / * ^"•***^ 03 ' f ^^^ 1 0 2 1 m E / / / . t . t . t . t . t T i 1 U1 0 100 2 0 0 3 0 0 4 0 0 500 D i s t a n c e , f t 6 0 0Figure 3-4: Variation in distance from a hydrogen fire for a thermal radiation exposure of 2 cal/cm2 and duration of 10 s (NASA, 2005)
The worst-case scenario resulting from a release of hydrogen is that of detonation and leads to the generation of a Shockwave and accompanying blast wave that have a significantly greater potential for damage or injury. The detonation limits of hydrogen is usually given as 18 - 59 vol. % in air and 1 5 - 9 5 vol. % in oxygen, but these values depend considerably on system size, geometry and confinement (12.5 vol.% in air has been observed). Therefore, the detonation limits cannot be specified for any fuel-oxidizer mixture unless the nature and dimensions of the confinement are also specified (NASA, 2005). To illustrate the effect that confinement has on detonation, Figure 3-5 shows the minimum dimensions required for detonation of hydrogen-air mixtures at 101.3 kPa for three types of confinement. It is clear that confinement severely influences the detonation limits and cloud dimension required. The critical tube diameter is the minimum diameter required for a detonation wave to emerge from a tube and become a detonation in an unconfined cloud. It is a measure of minimum dimensions of an unconfined detonable cloud and is usually in the range of 10 - 30 detonation cell widths (Verfondem & Nishihara, 2004a; NASA, 2005; BRHS, 2007).
Figure 3-5: Minimum dimensions of gaseous hydrogen-air mixtures for detonation at NTP (NASA, 2005)
The size of the detonation cell is a measure of its reactivity and is of considerable value in predicting the onset of detonation and describing the conditions for stable detonation waves (NASA, 2005; BRHS, 2007). Moreover, the cell size is related to
several key parameters used to assess a potential hazard, including critical energies and dimensional characteristics of structural confinement. The detonation cell size of hydrogen is 15.9 mm in a stoichiometric hydrogen-air mixture and 0.6 mm in a stoichiometric hydrogen-oxygen mixture at 101.3 kPa (NASA, 2005). Figure 3-6 illustrates the detonation cell size of hydrogen, methane, ethylene and acetylene in air at 101.3 K. 1000 E N c o a> Q 100 1 0 -Acetylene 10 20 30 40
% Fuel in air [%vol]
50 60
Figure 3-6: Detonation cell size of hydrogen, methane, ethylene and acetylene in air at 101.3 K(BRHS, 2007)
From this figure, it is clear that cell sizes increase with increasing deviation from stoichiometry and that hydrogen is the most reactive and methane (approximately 330 mm) is the least reactive of the common fuels (BRHS, 2007).
The detonation initiation energy is the minimum energy required to initiate a spherical detonation wave and depend on the concentration of the detonable mixture (NASA, 2005; BRHS, 2007). Figure 3-7 shows the minimum initiation energies for direct detonation of hydrogen-air mixtures at 101.3 kPa. Note that the axial unit in the y direction of Figure 3-7 (g tetryl) corresponds to the compound N,2,4,6-Tetranitro-N-methylaniline that was a common detonation compound in the previous century and therefore, it is occasionally used as a energy equivalent measurement of detonation energy (heat of detonation of tetryl is 12.24 MJ/kg).
Figure 3-7: Minimum initiation energies for direct detonation of hydrogen-air mixtures at 101.3 kPa (NASA, 2005)
The propagation of hydrogen explosions is extremely fast and relates to a detonation velocity of hydrogen in air of approximately 2000 m/s, while hydrogen-oxygen mixtures can reach 3500 m/s (Verfondern & Nishihara, 2004a). In comparison, high speed deflagrations have a flame speed in the order of 250 - 300 m/s and reach a maximum of 975 m/s in stoichiometric hydrogen-air mixtures (Piera eta/., 2006).
The detonation inductance distance is the distance in which the flame front at the ignition point develops into a detonation and depends on parameters such as temperature, pressure, mixture composition, geometry (obstacles) and ignition source strength. A typical ratio of tube length to diameter of approximately 100 is applicable for a stoichiometric hydrogen-air mixture (BRHS, 2007).
A hydrogen explosion caused by the ignition of a detonable hydrogen-air mixture releases approximately 0.1 - 10 % of its thermal energy content (usually less than 1%) and results in the formation of a pressure wave (Verfondern & Nishihara, 2004a; BRHS, 2007). The magnitude of the pressure wave is dependent on the detonation mode and differs significantly from unconfined, partially confined and confined detonations. The deflagration of a free hydrogen-air cloud results in a maximum overpressure in the order of 10 kPa (at 7 kPa people would fall down, 35 kPa eardrums would burst and 240 kPa is the threshold value at which fatalities would
occur). A fast and strong deflagration (flame speed < 340 m/s) could generate an overpressure of up to one bar, while detonations can reach overpressures of up to 20 bar (Piera et al., 2006; INSC, 2004). Figure 3-8 illustrates the pressure signals (overpressures) obtained from slow- and fast-speed deflagrations and detonations as a function of time (since ignition).
18 -, 18 -, 18 -, Sln\iv 16 • 14 •
Fast
—
Detonation
12 i CL 8 6 \ 4 \k
2]
\ > * * ~ ■ ' pm 0 0 1 0.1t, s
1 10Figure 3-8: Pressure signals from different hydrogen combustion modes (Lelyakin, 2005 as given in BRHS, 2007)
Even though the characteristics of hydrogen are such that it seems to be more dangerous than other combustible hydrocarbons, it has many safety advantages and several studies have found that hydrogen is no more, or less dangerous than any other fuel (Cadweller & Herring, 2007). Hydrogen's main advantage lies at its fast diffusion rate, which effectively reduces its hazardous potential in unconfined spaces such as those that will be used in most of the nuclear hydrogen production technologies under investigation. The following table (Table 3-1) is a summary of the physical and chemical properties of hydrogen.
Table 3-1: Physical and chemical parameters of hydrogen (BRHS, 2007)
S^OSgggiaSBg^B^B^^^tv;^;^-:?/;^-,-- :"■■?;: vf/*;<^
^ ^ f P I J f ^ f f p ^ f f l l ^
Buoyant velocity [m/s] 1.2-9 Specific heat (constant p) of gas @ NTP [kJ/(kg K)]
gas@STP[kJ/(kgK)] gas @ BP [kJ/(kg K)] liquid© BP[kJ/(kgK)] 14.85 14.304 12.15 9.66 Thermal conductivity of gas @ NTP [W/m K]
gas @ BP [W/m K] liquid @ BP [W/m K]
0.187 0.01694 0.09892 Viscosity of gas @ NTP [uPoise]
gas@ BP [uPoise] liquid @ BP [uPoise] 89.48 11.28 132.0 Surface tension @ BP [N/m] 1.93*10"3
Heat of conversion from para to ortho [kJ/kg] 708.8 Heat of melting (fusion) @ MP [kJ/kg] 58.8 Heat of vaporization @ BP [kJ/kg] 445.6 Vaporization index [K cm3/J] 8.9 Vaporization rate of LH2 pool [mm/s] 4.2 - 8.3 Heat of sublimation [kJ/kg] 379.6 Speed of sound in gas @ NTP [m/s]
gas @ BP [m/s] liquid @ BP [m/s]
in stoichiometric H2-air mixture [m/s]
1294 355 1093 975 Inversion temperature [K] 193 Flammability limits in air [vol.%] 4.0 - 75.0 Detonation limits in air [vol.%] 18.3-59.0 Minimum ignition energy [J] for detonation 1.9*10"i ~ 10,000
Auto-ignition temperature in air [K] 793-1023(858) Hot air jet ignition temperature [K] 943
Gross heat of combustion or HHV [kJ/mol] <H> 15 °C 286.1 Net heat of combustion or LHV [kJ/mol] @ 15 °C 241.7 Flame temperature [K] 2318 Burning rate of LH2 pool [mm/s] 0 . 5 - 1 . 1
Laminar burning velocity in air [m/s] 2.65-3.25 Flame speed [m/s] 18.6 Deflagration pressure ratio 8.15 Quenching distance @ NTP [mm] 0.64 Maximum experimental safe gap @ NTP [mm] 0.08 Adiabatic flame temperature [K] 2318
Detonation velocity [m/s] 1480 - 2150 CJ velocity [m/s] 1968 CJ detonation pressure ratio 15.6 Energy release [MJ/kg mixture] 2.82 Detonation cell size [mm] 15 Critical tube diameter [m] 0.2 Detonation initiation energy [g tetryl] 1.1
Detonation induction distance @ NTP Length/Diameter ~ 100 Critical explosion diameter [m] 0.16
3.3 METHANE (CH
4)
3.3.1 PHYSICAL PROPERTIES
Methane is a colourless, odourless, non-toxic, and non-acidic gas. It is flammable and detonable and may cause asphyxiation by displacing the oxygen in the atmosphere. Even though methane is considered the second most relevant greenhouse gas (GHG) after C02, accidentally releasing it into the atmosphere does
not have a significant impact on the local environment (Verfondern & Nishihara, 2004a).
Compared to hydrogen, methane gas under normal conditions has a moderate buoyancy and much smaller diffusion velocity and therefore, despite its low value of upper flammability limit, the lifetime of a flammable methane gas cloud is comparatively long (Verfondern & Nishihara, 2004a).
The storage of methane gas at very high pressures requires very strong storage tanks and the inadvertent release thereof may pose a fire hazard and even result in ejecting metal projectiles at very high velocities. The rapid expansion of methane gas from a highly pressurized container has a significant cooling effect on the gas and results in forming a vapour of very cold and dense gas that may linger near to the ground if a significant amount of the gas is released. Conventional designs assume that the gas released from a pressurized container will rise immediately (methane at atmospheric conditions is positively buoyant) and generally focus on ceiling ventilation and methane detection. However, if a significant amount of gas is released from a highly pressurized container, the cooling effect may result in a vapour that migrates downward and fills the low-lying areas. The methane will eventually warm up and rise, but it is extremely difficult to determine the time and configuration associated with the released methane and resulting flammable methane-air mixture during this warm-up period. Some of the chemical and physical properties of methane, hydrogen and carbon monoxide (CO) are given in Table 3-2 and even though most of hydrogen's properties are given in the previous table, some of it is repeated such that it can be compared to that of methane and carbon monoxide (Verfondern & Nishihara, 2004a).
Table 3-2: Safety related chemical and physical properties of CH4, H2 and CO
(Verfondem & Nishihara, 2004a)
f ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ S i # S i t . ; K ■
\-.r-'-:f$ei^^ft3fyMiiiiii
ilSfSEG^-?:-.Molecular weight [g/mol] 16.043 2.01594 28.01 Stoichiometric fraction in air [vol.%] 9.48 29.532 9.53 BP[K] 111.632 20.268 81.7 Melting point [K] 90.7 13.8 74.2 Density of liquid @ BP [kg/m3] 422.5 70.78 788.6 Density of gas @ BP [kg/m3] 1.82 1.338 4.355 Density @ STP [kg/m3] 0.7174 0.0899 1.25 Expansion ratio liquid/ambient 649 845 700 Diffusion coefficient @ STP [m2/s] 0.16+10"4 0.61+10"4
-Diffusion velocity [m/s] < 0.0051 <0.02 -Buoyant velocity [m/s] 0 . 8 - 6 1.2-9 -Heat of vaporization [kJ/kg] 509.9 445.6 215.2 Vaporization index [K cm3/J] 0.87 8.9 -Flammability limits in air [vol.%] 5.3-15.0 4.0-75.0 1 2 . 5 - 7 4 . 2 Detonable limits in air [vol.%] 6.3-13.5 18.3-59.0 -Minimum ignition energy [mJ] 0.29 0.019 -Auto-ignition temperature in air [K] 903 - 1 4 9 3 793 - 1 0 2 3 878 Flash point [K] - - -Gross heat of combustion [kJ/mol] @ 15°C 891.5 286.1 282.9 Net heat of combustion [kJ/mol] @ 15°C 803.3 241.7 282.9 Laminar burning velocity in air [m/s] 0.37 - 0.45 2.65-3.25 0.33 Flame front velocity [m/s] 3.2 18.6 0.52 Quenching distance [mm] 2.03 0.64 -Maximum experimental safe gap [mm] 1.2 0.08 -Adiabatic flame temperature [K] 2148 2318 2370 Detonation velocity [m/s] 1390 -1640 480 - 2150 -a velocity [m/s] 1801 1968 -a deton-ation pressure r-atio 17.2 15.6 -Energy release [MJ/kg mixture] 2.31 2.82 -Detonation cell size [mm] 250 - 310 15 -Critical tube diameter [m] - 0.2 -Detonation initiation energy [gtetryl] 22,000 1.1 -Critical explosion diameter [m] 4 0.16 -TNT equivalent [g-TNT/g] 11.1 26.5
-3.3.2 CHEMICAL PROPERTIES
Methane is a very flammable gas and has a flammability limit of 4.4 - 1 6 . 5 vol. % . in air and 5 - 6 1 vol. % in oxygen, with the maximum amount of energy released at its stoichiometric concentration of 9.4 vol. % in air. The lower flammability limit of
methane is comparable to that of hydrogen and is similarly dependent on temperature (7.7 % at 112 Kand 2.3 % at 723 K). Moreover, if the initial temperature of the mixture is decreased, the lower flammability limit increases (20 % at 150 K). Figure 3-9 shows the flammability diagram for methane-oxygen-nitrogen mixtures at atmospheric pressure and 26 °C (Verfondem & Nishihara, 2004a).
Figure 3-9: Flammability diagram for methane-oxygen-nitrogen mixtures at atmospheric pressure and 26 °C (Verfondem & Nishihara, 2004a)
Similar to hydrogen, increasing pressure widens the flammability range, while addition of inert gases such as nitrogen or carbon dioxide narrows it. Inert gases need only be added until suppression of the flame is reached, which is at 38 % nitrogen or 24 % carbon dioxide for methane-air mixtures (Verfondem & Nishihara, 2004a).
The combustion of methane results in a bluish flame and produces water and carbon dioxide according to the following reaction:
CH4. + 202^>C02 + 2H20 (-979.6 kJ/mol) Reaction 3-2
Methane has an auto-ignition temperature of 813 K and minimum ignition energy of 0.29 mJ. These values are somewhat higher than that of hydrogen, and the minimum ignition energy is still readily achieved by a weak spark or electrostatic discharge. No flash point or fire point is given for methane (Verfondern & Nishihara, 2004a).
The energy released when a free methane-air mixture combusts is not expected to be higher than 10 % of its total thermal energy content before combustion. This value corresponds to that of hydrogen, which is in the range of 0.1 - 10 % (Verfondern & Nishihara, 2004a).
Methane thermally decomposes at temperatures above 1373 K and requires at least 12 vol. % oxygen to propagate a flame. The maximum burning velocity of a methane-air mixture at ambient conditions is significantly smaller than the corresponding value of hydrogen (0.37 - 0.45 m/s vs. 2.65 - 3.25 m/s). The maximum burning velocity of methane in oxygen is 3.9 m/s, while it is 11.75 m/s in hydrogen-oxygen mixtures. The flame speed of a stoichiometric methane-air concentration ranges from 400 - 800 m/s, which is also smaller than that of hydrogen (max. 975 m/s). These values indicate that methane has a lower probability to experience a DDT or detonation than hydrogen (Verfondern & Nishihara, 2004a).
The quenching distance and maximum experimental safe gap of methane are 2.03 mm and 1.2 mm respectively. These values are significantly higher than that of hydrogen (0.64 mm and 0.08 mm) and are an indication of hydrogen's extreme combustion capabilities (Verfondern & Nishihara, 2004a).
The flame temperature of methane at stoichiometric concentration with air (2148 K) is somewhat lower than the corresponding value of hydrogen (2318 K). However, since the emissivity of a methane flame is significantly higher than that of hydrogen (e ~ 1 vs. s = 0.1), much more heat is radiated to the surroundings by a methane flame and therefore require larger separation distances. The overpressures resulting from deflagrations of equivalent quantities of fuel are significantly weaker for methane than for hydrogen (Verfondern & Nishihara, 2004a).
3,4 NATURAL GAS
Natural gas is a colourless, odourless and relatively non-toxic gaseous mixture of methane with small amounts of heavier hydrocarbons (ethane, propane, butane and etcetera) and marginal amounts of N2, 02, C02, H2S, He, and H20 of varying shares.
The composition of natural gas varies significantly from one source to another and results in considerable variation of its general physical properties. Table 3-3 shows the components and their typical ranges of contribution in pipeline natural gas.
Table 3-3: Composition of natural gas (NaturalGas, 2007)
<§ssfijs®»a»£. -..; V '.; '.(Wtefl^o^^witf;>
Wi«^^^^^^s^i
Methane CH4 70-90% Ethane C2H6 0-20% Propane C3H8 0-20% Butane C4H10 0-20% Carbon Dioxide co2 0-8% Oxygen o2 0-0.2% Nitrogen N2 0-5% Hydrogen sulphide H2S 0-5%
Rare gases A, He, Ne, Xe trace
Natural gas that consists primarily of methane (after being processed) is known as dry natural gas, whereas natural gas containing a significant amount of heavier hydrocarbons is termed wet natural gas. Moreover, natural gas containing sulphur impurities (hydrogen sulphide) is termed sour gas while natural gas containing sulphur and carbon dioxide is acid gas. The amount of heavier hydrocarbons and impurities such as hydrogen sulphide, carbon dioxide, and mercury (present in some of the sources) play a fundamental role in the safety of natural gas. While dry natural gas is lighter-than-air and diffuses into the atmosphere, wet natural gas is heavier-than-air and tends to linger near the ground, thereby increasing the probability of ignition and explosion. The heavier hydrocarbons additionally increase the energy released upon deflagration or detonation. All forms of natural gas are an explosion hazard in partially confined and confined geometries where the gas can accumulate underneath an obstacle and reach flammable and detonable concentrations (Verfondern & Nishihara, 2004a).
While processed natural gas (dry) is a simple asphyxiant (displaces air and oxygen), the untreated gas contains hydrogen sulphide and is therefore toxic. Moreover, some
fields additionally contain mercury, which is poisonous. The organo-sulphide compounds of hydrogen sulphide and carbon dioxide are corrosive substances, especially in the presence of water, and become more corrosive as the pressure increases (Verfondern & Nishihara, 2004a). The flammability limit of natural gas varies according to its composition but is in the range of 5 - 15 vol. % in air. It requires a minimum ignition energy in the range of 0.15 - 0.30 mJ in air and has an auto-ignition temperature at approximately 450 - 500 °C (Verfondern & Nishihara, 2004a).
3.5 CARBON MONOXIDE (CO)
Carbon monoxide is a colourless, odourless, tasteless, highly toxic and flammable gas and requires special precautions in handling and storage (Verfondern & Nishihara, 2004a). Its toxicity to humans and animals stems from its extreme affinity for haemoglobin (210 - 240 times greater than that of oxygen), which is responsible for the transport of oxygen in blood. Carbon monoxide reacts with haemoglobin to form carboxy-haemoglobin, which is not able to transport oxygen, but which is a reversible process (increase the oxygen levels in blood by inhaling pure oxygen). Carbon monoxide poisoning attacks the central nervous system (CNS) and heart and is the most common type of fatal poisoning in many countries. Carbon monoxide additionally damages the red blood cells (haemolytic poison). Symptoms of mild poisoning include headaches and dizziness at concentrations less than 100 ppm. Concentrations as low as 667 ppm can be life threatening since up to 50 % of the body's haemoglobin can be converted to carboxy-haemoglobin at this concentration. An atmosphere containing 0.2 % of carbon monoxide is lethal after about 2 hours. It is a chemical asphyxiant with a recommended threshold limit of 0.01 % in air (Verfondern & Nishihara, 2004a). Long-term workplace exposure levels must be limited to 50 ppm (OSHA) or 3760 ppm/hour (Air Liquide MSDS: Carbon monoxide).
Carbon monoxide is produced from the partial combustion of carbon-containing compounds due to insufficient oxygen content. Carbon monoxide has significant fuel value and bums in air with a non-luminous blue flame to produce carbon dioxide. It has and an auto-ignition temperature of 620 °C and flammability limits of 12.5 - 74 vol. % in air and 1 5 - 9 4 vol. % in oxygen. Carbon monoxide reacts violently with oxidizers, has a detonation limit of 38 - 90 vol. % in oxygen and a maximum detonation velocity of approximately 2800 m/s in oxygen (Verfondern & Nishihara, 2004a). A leaking flame should rather be allowed to burn out than be extinguished
since spontaneous and explosive re-ignition may occur. Carbon monoxide is slightly lighter-than-air at ambient conditions and will therefore slowly diffuse into the surrounding atmosphere (Verfondem & Nishihara, 2004a).
Carbon monoxide causes no known ecological damage but reacts with steel at high temperatures to produce small quantities of iron carbonyl. It is neither corrosive nor an irritant and is rapidly oxidized to C02 (Verfondem & Nishihara, 2004a).
3.6 CARBON DIOXIDE (C0
2)
Carbon dioxide is an inert, non-flammable, colourless, and odourless gas (refer to Table 3-4 for characteristic properties). It is non-toxic at physiological concentrations but causes rapid circulatory insufficiency at higher concentrations. The toxicity of carbon dioxide according to the Australian Maritime Safety Authority is:
"Prolonged exposure to moderate concentrations can cause acidosis and adverse effects on calcium phosphorus metabolism resulting in increased calcium deposits in soft tissue. Carbon dioxide is toxic to the heart and causes diminished contractile force. At concentrations of three percent by volume in air, it is mildly narcotic and causes increased blood pressure and pulse rate, and reduced hearing. At concentrations of about five percent by volume it causes stimulation of the respiratory centre, dizziness, confusion and difficulty in breathing accompanied by headache and shortness of breath. At about eight percent concentration it causes headache, sweating, dim
vision, tremor and loss of consciousness after exposure for between five and ten minutes".
Due to the health risks associated with carbon dioxide inhalation, the OHSA limits the average exposure for healthy adults during an eight-hour workday to 0.5 %. Short-term (under ten minutes) exposure is limited to 3 % (NIOSH and ACGIH), while concentrations exceeding 4 % are immediately dangerous to life and health (Air Liquide MSDS: Carbon dioxide). Carbon dioxide is heavier-than-air and may accumulate in confined spaces, particularly at or below ground level where it may lead to asphyxiation at high concentrations. Ecologically, carbon dioxide is a greenhouse gas (GHG) and when discharged to the atmosphere in large quantities will contribute to the greenhouse effect (Air Liquide MSDS: Carbon dioxide).
Table 3-4: Characteristic properties of C02, CO, HI, CH4, S02, S03 and H2SO„ Component Carbon Dioxide Carbon Monoxide Hydrogen Iodide Methane Sulphur Dioxide Sulphur
Trioxide Sulphuric Acid Molecular Formula
co
2 CO HI CH4so
2 S03 H2SO4Structure
Jfr
• »
e
«t
O
tt
CAS number 124-38-9 630-08-0 10034-85-2 74-82-8 7446-09-5 7446-11-9 7664-93-9
Appearance Colourless gas Colourless,
odourless gas Colourless gas Colourless gas
Colourless gas, pungent odour Clear, colourless liquid Clear, colourless, odourless liquid.
Molar mass 44.0095 g/mol 28.0101 g/mol 127.904 g/mol 16.0425 g/mol 64.054 g/mol 80.06 g/mol 98.078 g/mol
Density 1.98 g/L 1.145 g/L at 25°C, 1 atm 2.85 g/mL (-47 °C) 0.717 kg/m3 2.551 g/L 1.92 g/cm3 1.84 g/cm3, liquid Melting Point -57°C(216K) (under pressure) -205 °C (68 K) -50.80 °C (184.55 K) -182.5 °C (91 K) -72.4 °C (200.75 K) 16.9 °C 10 °C, 283 K BP -78°C(195K) -192°C(81 K) -35.36 °C (237.79 K) -161.6 °C, 112 K -10°C(263K) 45 °C 290 °C (563 K) Solubility in H20 1.45g/Lat25°C, 100kPa 0.0026 g/100 mL (20 °C) 3.5mg/100mL (17 °C) 9.4 g/100 mL (25 °C) Hydrolysis Fully miscible (exothermic) Acidity (pK,) 6.35 and 10.33 -10 1.81
Viscosity 0.07cPat-78°C 26.7cPat20°C
Hazard Toxic at higher
concentrations Highly Flammable and Toxic Highly Toxic. Corrosive Highly
Flammable Highly Toxic Corrosive Highly Corrosive
NFPA 704
(SEE APPENDIX A FOR RATINGS)
#
^ O R X
Table 3-5: Characteristic properties of H2, l2, 02 and He
Component Hydrogen Iodine Oxygen Helium
Molecular Formula H2 l2
o
2 HeStructure
^
§)
( ) Mono-atomicPhysical state at 20 °C Compressed gas Solid (Crystals) Compressed gas Compressed gas
Colour Colourless Purple to black Colourless gas Colourless gas
Odour None Strong characteristic odour No odour warning No odour warning
Molecular weight [g/mol] 2 253.81 32 4
Melting point [°C] -259 113.5 -219 Not applicable
BP[°C] -253 184 -183 -269
Critical temperature [°C] -240 -118 -268
Relative density, gas (air = 1) 0.07 9 1.1 0.14
Relative density, liquid (water = 1) 0.07 1.1 Not applicable
Solubility in water [mg/(] 1.6 Soluble 39 1.5
Flammability range [vol.% in air] 4 to 75 Not applicable Oxidizer Non flammable
Auto-ignition temperature [°C] 560 Not applicable Not applicable Not applicable
Other data Burns with a colourless
invisible flame.
Conditions of Instability: Heat, direct sunlight, incompatible
materials.
Gas/vapour heavier-than-air. May accumulate in confined spaces, particularly at or below ground level.
Stability & Reactivity
Can form explosive mixture with air. May react violently with
oxidants.
Reactive with oxidizing agents, reducing agents, metals. Extremely corrosive in presence
of steel.
May react violently with combustible materials. May react
violently with reducing agents. Violently oxidizes organic material.
Stable under normal conditions.
Toxicity None Toxic and corrosive. None None
Ecological Information None Very toxic to aquatic organisms. None None
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3.7 SULPHURIC ACID (H
2S0
4)
Sulphuric acid is a clear, colourless, odourless, non-flammable but acid-tasting and highly corrosive liquid. It is very hazardous in case of skin contact (corrosive, irritant, permeator), of eye contact (irritant, corrosive), of ingestion, and of inhalation. Skin contact may produce bums, while inhalation causes severe irritation of the respiratory tract that is characterized by coughing, choking, or shortness of breath. Sulphuric acid has an exposure limit of 1 mg/m3 (NIOSH, ACGIH) and severe
over-exposure can result in death. Concentrations between 0.6 and 1.5 mol/L are labelled IRRITANT, with concentrations exceeding 1.5 mol/L are labelled CORROSIVE. Additionally, sulphuric acid is a suspected human carcinogen (contained in strong inorganic acid mists).
The corrosive properties of sulphuric acid are accentuated by its highly exothermic reaction with water to form toxic and corrosive fumes. The violent liberation of heat when sulphuric acid is exposed to water may result in the splattering of material and containment rupture due to the build up of pressure. Because the hydration of sulphuric acid is thermodynamically favourable, sulphuric acid is an excellent dehydrating agent.
Although sulphuric acid is non-flammable, contact with metals in the event of a spillage can lead to the production of hydrogen gas, which is highly flammable and detonable. Additionally, fires involving sulphuric acid can result in the dispersal of acid aerosols and gaseous sulphur dioxide, and the presence of sulphuric acid in a fire may accelerate the burning process.
Decomposition of sulphuric acid occurs at temperatures exceeding 340 °C to produce oxides of sulphur (irritating) and toxic fumes and gases. Sulphuric acid reacts with carbonates, cyanides and sulphides to form carbon dioxide, poisonous hydrogen cyanide, and hydrogen sulphide respectively. It is extremely corrosive in presence of aluminium, copper, and stainless steel (316), and highly corrosive in presence of stainless steel (304).
In its liquid state (BP of 270 °C), sulphuric acid is heavier than water (SG = 1.84), while in its gaseous state it is significantly heavier-than-air (by a factor of at least 4). Therefore, gaseous sulphuric acid will accumulate at, or below ground level until it
condenses to a liquid state or react with other material. This is a significant hazard to both operating personnel and equipment due to its strong corrosive properties.
3.8 SULPHUR DIOXIDE (S0
2)
Sulphur dioxide is a non-flammable and colourless gas with pungent odour that causes severe corrosion to skin, eyes and respiratory tract at high concentrations. It is toxic by inhalation and results in laboured breathing, coughing, and sore throat and may cause permanent pulmonary damage. Delayed fatal pulmonary oedema is possible with prolonged exposure at high concentrations (LC50: 2520 ppm/hour). Frostbite may additionally occur when sulphur dioxide mixes with water and makes contact with skin (Air Liquide MSDS: sulphur dioxide).
Although sulphur dioxide is non-flammable, it reacts with most metals in the presence of water to produce hydrogen, which is extremely flammable. It reacts violently with alkalis and is extremely corrosive to some metals in the presence of water. When mixed with water, hydrolysis occurs to form corrosive acid solutions (Air Liquide MSDS: sulphur dioxide).
From an environmental perspective, the oxidation of sulphur dioxide produces sulphuric acid and thus acid rain. Therefore, avoid the release of sulphur dioxide into the atmosphere. Furthermore, sulphur dioxide may cause pH changes in aqueous ecological systems due to the hydrolysis process (Air Liquide MSDS: sulphur dioxide).
Similar to sulphuric acid, sulphur dioxide gas is heavier-than-air and may accumulate in confined spaces, particularly at or below ground level, thereby increasing the corrosion, toxic and ecological hazard (Air Liquide MSDS: sulphur dioxide).
3.9 SULPHUR TRIOXIDE (S0
3)
Sulphur trioxide is a clear and colourless liquid with a melting point at 16.8 °C and BP at 44.7 °C. It is very toxic by inhalation, is severely corrosive to mucous membranes, skin and eyes, and is a proven human carcinogenic. The recommended exposure limit is 1 - 3 mg/m3, while prolonged exposure to higher concentrations may be fatal
Water hydrolyzes sulphur trioxide to sulphuric acid gas, which in contact with metal surfaces can generate flammable and/or explosive hydrogen gas. Sulphur trioxide vigorously supports combustion and may cause fires when in contact with combustible material. Moreover, it emits toxic fumes under fire conditions (Air Liquide MSDS: Sulphur trioxide).
Although pure sulphur trioxide is not corrosive to metals, it reacts violently with water to form sulphuric acid, which is extremely corrosive. Furthermore, sulphur trioxide may undergo hazardous decomposition, condensation, or become self-reactive under conditions of shock or increased temperatures or pressures (Air Liquide MSDS: Sulphur trioxide).
When sulphur trioxide makes contact with air, it rapidly absorbs the ambient moisture to release dense, white sulphuric acid fumes. Its affinity for water and the subsequent release of sulphuric acid is of major concern to aqueous ecological systems. Moreover, sulphur trioxide is the primary agent in acid rain and therefore any release into the atmosphere should be avoided (Air Liquide MSDS: Sulphur trioxide).
Sulphur trioxide is heavier-than-air and will accumulate in the low-lying areas after being released. Similar to the other components that are heavier-than-air, this accumulation increases the potential of the hazards associated with sulphur trioxide (Air Liquide MSDS: Sulphur trioxide).
3.10 IODINE (I2)
At room temperature, iodine is a purple-black solid with metallic lustre that slowly sublimes into a violet-pink gas with a sharp characteristic odour. It is a severe irritant, corrosive to the eyes, skin, mucous membranes and respiratory tract, and toxic if inhaled or swallowed. Iodine is readily absorbed through the skin and may be fatal if 2 - 3 g are ingested or inhaled. Short-term exposure is limited to 0.1 - 1.1 ppm depending on the local authorities.
Iodine is not flammable but toxic vapours are liberated when it is subjected to flames or other sources of heat. Sublimation of iodine also yields toxic vapours, which may reach dangerous concentrations when confined. Iodine is extremely corrosive in the
presence of steels but non-corrosive in the presence of glass, aluminium, copper, or bronze.
Iodine is an oxidizer and contact with combustible materials should be avoided. Although generally non-flammable, it ignites under certain conditions including when in contact with bromine, fluorine, magnesium and powdered metals with water present. Iodine is very toxic to aquatic organisms and may cause long-term harm to the environment. Iodine and its vapours are heavier-than-air and may accumulate in confined spaces, particularly at or below ground level.
3.11 HYDROGEN IODIDE (HI) AND HYDRIODIC ACID
Hydrogen iodide is a non-flammable and colourless gas with pungent odour. It is toxic by inhalation and very corrosive to the eyes, respiratory system and skin. Hydrogen iodide is fatal at high concentrations by causing delayed fatal pulmonary oedema (LC50 = 2860 ppm/hour).
Hydrogen iodide reacts with oxygen to form water and iodine. If it is exposed to air, hydrogen iodide will undergo the following reactions:
4HI + 02^> 2H2O + 2I2 Reaction 3-3
HI + I2-> HI3 Reaction 3-4
Furthermore, when it is released into the atmosphere, hydrogen iodide quickly dissolves into the ambient moisture to form a white mist of hydriodic acid. Aqueous solutions of hydrogen iodide are known as hydriodic acid, which is a very strong acid. Hydrogen iodide is exceptionally soluble in water and one litre of water will dissolve 425 litters of hydrogen iodide, with the final solution having only four water molecules per molecule of hydrogen iodide. Additionally, hydrogen iodide and hydriodic acid are inter-convertible.
Hydriodic acid is much stronger than most of the other common halide acids and hydrogen iodide is the strongest acid of the hydrohalides with a Ka value of
approximately 1010 (HBr: Ka ~ 109; HCI: Ka = 108). Since both hydrogen iodide and
Similar to the previously discussed compounds, neither hydrogen iodide nor hydriodic acid is flammable, but release flammable hydrogen gas when reacting with metals in the presence of water. Hydriodic acid is extremely corrosive to most metals and necessitates proper material selection. Furthermore, hydriodic acid has an azeotrope at 127 °C consisting of 57 % hydrogen iodide and 43 % water, which makes separation by distillation impossible beyond the azeotrope.
Hydrogen iodide and hydriodic acid are hazardous to the environment due to causing pH changes in aqueous ecological systems, which renders the water acidic and unable to sustain life. Both hydrogen iodide and hydriodic acid are heavier-than-air and may accumulate in confined spaces, particularly at or below ground level.
3.12 OXYGEN (02)
Oxygen is a colourless, odourless, tasteless, non-toxic and non-flammable gas, as well as an oxidizing material that vigorously supports combustion and may cause oxygen toxicity at high concentrations (Air Liquide MSDS: Oxygen).
Oxygen toxicity is severe hyperoxia caused by breathing oxygen at elevated partial pressures and can cause cell damage to the central nervous system (CNS), lungs (pulmonary) and retina. The damage may be caused by either long exposure (days) to lower oxygen concentrations or by shorter exposure (minutes or hours) to high oxygen concentrations. Long exposures to partial pressures of oxygen above 50 kPa may result in pulmonary oxygen toxicity, while short exposures to partial pressures of oxygen above 160 kPa may result in CNS oxygen toxicity. Prolonged exposure to high oxygen concentrations causes damage to the retina. Continuous inhalation of oxygen concentrations higher than 75% may cause nausea, dizziness, respiratory difficulty and convulsion (Air Liquide MSDS: Oxygen).
Oxygen powerfully supports combustion and may react violently with combustible materials or reducing agents. High concentrations of oxygen will allow combustion to proceed rapidly and energetically, thereby increasing the flammability and detonation hazard associated with combustible material. To this extent, it would be wise to separate the storage of oxygen and fuel (hydrogen) by a significant distance if oxygen is produced as by-product during the (nuclear) production of hydrogen.
Oxygen is heavier-than-air and may accumulate in confined spaces, particularly at or below ground level, thereby increasing the risk of flammability and detonation with nearby combustible material. Oxygen has no ecological or toxicological effects but may violently oxidize organic material (Air Liquide MSDS: Oxygen).
3.13 HELIUM (He)
Helium is a colourless, odourless, tasteless, non-toxic, inert monatomic chemical element and is the lightest of the noble gases. It has the lowest boiling and melting points among the elements and only exists as a gas (theoretically under extreme conditions).
Helium at high concentrations may cause asphyxiation by reducing the oxygen content of the air. However, helium is considerably lighter-than-air (relative density of 0.14) and may quickly dissipate into the atmosphere if not confined. Symptoms of asphyxiation include loss of mobility or consciousness and may occur while the victim is not aware of it. Helium is stable under normal conditions and has no known toxicological effects or ecological damage (Air Liquide MSDS: Helium).
3.14 CONCLUDING REMARKS
Since this section dealt with the identification of hazards as a consequence of the substances present at the production facility, the next issue to be discussed is the ways in which these hazards occur and propagate. Considering the hazardous substances discussed in this section, the major propagation methods to be investigated are the evolution of gaseous clouds, ignition and combustion of flammable gas clouds, the consequences associated with combustion, and hydrogen embrittlement. If the propagation method is known and understood the attendant risk to the nuclear facility can be evaluated. The criteria of measuring the extents of hazards according to their risk to the nuclear facility are based on the relative consequences of a nuclear accident as opposed to that of a chemical accident. Moreover, the regulations and requirements of nuclear plants are significantly stricter than that of chemical facilities and are expected to be barriers to the implementation of a nuclear/chemical complex. Therefore, the propagation methods or accident phenomena associated with the hazardous substances present at the chemical facility form the next topic of discussion.
